This invention relates generally to power generating equipment and, more particularly, to engine-driven generators.
Known portable power generators typically include an internal combustion engine turning an alternator designed to provide a 120 or 240 volt, 50 or 60 hertz alternating current output. One characteristic of such prior power generators is that, in use, the engine speed is held substantially constant regardless of the actual load. This is necessary because, in such generators, the output frequency is a direct function of engine speed. Running the engine at a constant (usually high) speed results in excessive noise, vibration and fuel consumption, particularly where the load is relatively light and maximum engine power is not required. Alternatively, if multiple pole alternators are used to permit the use of lower synchronous engine speeds, considerable unused engine capacity is frequently available. This is wasteful and makes the generator larger, heavier and more expensive than it needs to be.
Another characteristic of prior power generators (particularly those using permanent magnet alternators) is that the alternators they include are not run at their maximum power output levels. At a given engine speed, such alternators typically provide maximum power when the alternator output voltage drops to about 70% of the no load voltage. Such a voltage drop, however, is unacceptable in practice, and the usual practice is to run alternators at far less than their maximum power output levels. Thus, to achieve a useful output power of, say, 3.5 KW with a voltage drop of only 5%, the usual practice is to use a permanent magnet alternator capable of developing far more than 3.5 KW. Such an alternator thus has considerable excess capacity that is never utilized, and is physically larger, heavier and more expensive than an alternator having a maximum power capability of 3.5 KW. The challenge, therefore, is to operate an alternator at up to its peak power capabilities while still keeping the output frequency and voltage within acceptable limits.
To improve efficiency and reduce size and weight, some prior generators have combined high frequency multiphase alternators with electrical circuitry for converting the high frequency multiphase alternating current to a 50 Hz or 60 Hz single-phase current independent of engine speed. Such a system is shown, for example, in U.S. Pat. No. 3,916,284 which issued Oct. 28, 1975 to Walter P. Hilgendorf. In the Hilgendorf system, voltage regulation is provided by varying the current through the pole field winding of an alternator having a polyphase armature winding and a self-excited field winding. Accordingly, the Hilgendorf system is not readily adapted for use in permanent magnet alternators having a fixed stator and a permanent magnet rotor arranged to rotate around the stator. Because permanent magnet alternators provide many advantages over self-excited alternators, this shortcoming of the Hilgendorf system is significant.
In view of the foregoing, it is a general object of the present invention to provide a new and improved engine-driven portable generator.
It is a further object of the present invention to provide a new and improved engine-driven portable generator that is economical, lightweight, efficient and quiet.
It is a further object of the present invention to provide a new and improved engine-driven portable generator that permits a permanent magnet alternator to be operated up to its maximum power output, while still providing acceptable and useful frequency and voltage regulation.